Lecture 11 - Rb and Control of the Cell Cycle Clock Continued... Flashcards
What determines the progression through a restriction point?
The level of pRb phosphorylation determines the progression through a restriction point.
Describe the phosphorylation levels of pRb throughout the cell cycle.
What is the relationship between certain tumor viruses and pRb?
Phosphorylation of pRb occurs in three stages throughout the cell cycle. At the beginning of G1, following cell division, pRb is completely dephosphorylated and the period between the early G1 phase and the end G1 restriction point is known as the hypophosphorylation stage, dealing with the D-cyclins. Following the end G1 restriction point, pRb is said to undergo hyperphosphorylation at an increasing scale until the M phase is reached. At the M phase, there is a rapid dephosphorylation event following cell division and the whole process starts again.
Certain tumor viruses contain genetic material that codes for certain oncoproteins that bind to pRb and influence its normal function in some way. DNA tumor virus proteins bind preferentially to hypophosphorylated Rb
Does the Rb protein contain a phosphate group at the very very beginning of G1 or is it completely P-group “nude”?
At the beginning of G1 the Rb protein is completely nude and unphosphorylated.
Describe the relationship between pRb and the E2F family of transcription factors throughout the stages of the cell cycle.
During early G1 and mid G1 phases, pRb is hypophosphorylated by stimulus of the D-cyclins CDK4/6 complexes binding together. This means that only a single phosphate group is bound to the protein, and it can therefore sequester the E2F transcription factors and hold them tightly. This discourages transcription as the DNA is wound up super tightly through ionic bonds (-ve charge) between acetyl and histone groups. Furthermore, HDAC (Histone deacetylase) removes the acetyl to further bunch up the DNA and prevent transcription.
Following the restriction point (late G1), the pRb becomes hyperphosphorylated (multiple P groups attach) by stimulus of the E-cyclin CDK2 complexes binding together. This causes the bending the shape of the protein and releasing the transcription factors from their captors.
No longer bound, the pRb molecule can no longer prevent the E2F transcription factors from functioning and transcribing certain genes.
How is the cell cycle progression of pRb and E2F a positive feedback loop?
The E CDK2 cyclin-CDK complex stimulates the hyperphosphorylation of pRb which releases the E2F transcription factor which, in turn, influences the increased synthesis of E CDK2 cyclin-CDK complexes.
This cycle cycles through a few times to amp up the phosphorylated pRb
What neat side effect do increased levels of cyclin E lead to?
Increased levels of cyclin E leads to the degradation of p27Kip1 (the inhibitor of S, G2, and M phase cyclin complexes)
Degradation via proteasome
What is the effect of Ras issues on the cell cycle clock?
Ras issues leads to both the accumulation and degradation of cyclin D1.
Can cause overexpression and simultaneous ubiquitylation (degradation) via proteasome.
How does Myc work in association with Max and Mxd? What are the effects of low and high Myc levels and how do these come about?
Myc can promote proliferation when associated with Max. Myc levels are strongly influenced by mitogenic singals.
When levels are high, the Myc-Max combo transcription factor causes mitogenic signalling and active proliferation.
When levels are low, Mxd takes Myc’s place in the now Mxd-Max combo, which causes the repression of mitogenic signalling and reduced proliferation.
What is the effect of Myc on the cell cycle clock? What does it promote? How does this occur?
The Myc-Max combo promotes proliferation by increasing D-cyclin CDK4/6 levels in early/mid G1.
Myc also binds with Miz-1 to block inhibitors such as p15INK4B (half-shell), p21Cip1 and p27Kip1.
Essentially:
Myc increases cyclin levels
Myc increases E2F levels
Myc decreases CDKi’s
What is the super-interesting secret superpower held by the Myc oncoprotein? How does it accomplish its function?
The Myc oncoprotein can trigger movement through a restriction point. It does so by binding with Max and an estrogen ligand (example) to bind to the genome in the nucleus and move past the late G1 restriction point.
What is the relationship between TGF-beta and Myc? How does Myc counterattack TGF-beta?
TGF-beta can inhibit E2F4/5 which inhibits Myc expression.
Myc can counter by inhibiting the expression of CDK inhibitors, effectively blocking the TGF-beta pathway.
What is interesting about many of the same factors that regulate the progression through the restriction point?
They also help regulate cell differentiation
Ex: muscle differentiation program
